Seven transmembrane-spanning receptors (7TMRs or G protein-coupled receptors, GPCRs) represent the largest family of signal-transducing molecules known. 7TMRs convey signals for light and many extracellular regulatory molecules, such as, hormones, growth factors and neurotransmitters, that regulate every cell in the body. Dysregulation of 7TMRs has been found in a growing number of human diseases and 7TMRs have been estimated to be the targets of more than 30% of the drugs used in clinical medicine today. Thus, understanding how 7TMRs function is an important goal of biological research. We have used receptors for thyrotropin-releasing hormone (TRH) (TRH-Rs) and for thyroid-stimulating hormone (TSH-R) as model 7TMRs to study their structure and function. During this year, we studied several new aspects of the structure and function of these receptors. 1) An important new observation for several 7TMRs is that they can continue signaling hours after their cognate agonist has been removed. This has been termed persistent signaling. We showed previously that TSHRs exhibit persistent signaling. During this year, we showed that the amplitude of persistent signaling correlates with the level of expression of TSHRs and their signal transducing G protein. These findings help explain the different levels of persistent signaling observed in different cell types. 2) We showed that an analog of TRH, taltirelin, which is the only TRH analog (other than TRH itself) approved for use in humans (although not in the USA), is a superagonist (an agonist that is more active than the cognate agonist, in this case TRH) at TRH-Rs. This helps explain, at least in part, why taltirelin is a more effective agonist in studies in animal models and in humans than is TRH. 3) There is a single TRH-R in humans (and several other species) but two subtypes (TRH-R1 and TRH-R2) in rodents. Based on the anatomical distributions of TRH-R1 and TRH-R2 in rodents, it had been proposed and accepted for many years that TRH-R1 mediated the endocrine effects of TRH, for example, stimulation of TSH secretion from the pituitary gland, and that TRH-R2 mediated the central nervous system effects of TRH, for example, its antinociceptive effects. We developed mice that did not express TRH-R1, TRH-R2 or both TRH-R1 and TRH-R2 and used these to show that the endocrine and the known CNS effects of TRH are mediated by TRH-R1. This was important because the single TRH-R in humans is much more like TRH-R1 than TRH-R2. This will allow us to focus on TRH-R1 in animal models in trying to understand the physiology and pharmacology of TRH receptors.